In electrochemical systems, salt- or simple-bridges have been widely employed as a means to isolate electrodes and electrode byproducts from the working fluid, or more generally to isolate one electrochemical environment from another while maintaining ionic communication. A common example is the porous tip of a pH probe. In electroosmotic flow systems, a bridge is used as an ionic conductor that separates the working fluid from the fluid that is in direct contact with the electrodes. The prior art discloses several types of bridges.
For example, Theeuwes discloses the use of membrane bridges between the electrodes and working fluid in an electroosmotic pump. F. Theeuwes, Electroosmotic pump and fluid dispenser including same, U.S. Pat. No. 3,923,426 (1975). The membrane material is a sulfonated polymer having a relatively high zeta potential and very fine pores. The Theeuwes device is essentially a double-reservoir design with the outer (electrode) and inner (working fluid) reservoirs separated by the membrane. The membranes are selected for a very high charge-ratio (defined infra) and selectivity to positive ions (for example, Ag+ and H+ in the Theeuwes case, which thus inhibit current-driven growth of silver dendrites on silver: silver-chloride electrodes).
Seoul discloses a double reservoir bridge design for use in capillary electrophoresis where the outer reservoir contains a platinum wire electrode embedded in a Nafion resin. B. S. Seoul, Electrochemical detection for capillary electrophoresis (Doctoral dissertation, U. Kansas, 1996) pp. 136–141. The objective is to minimize the degradation of analytes by peroxides generated at the anode electrode. Nafion is a sulfonated fluorocarbon polymer that is either solid or very fine pored. It acts as an ionic conductor that is highly selective to positive ions and thus exhibits a very high charge-ratio, implying that current through this material is essentially carried solely by transport of positive ions. This is quite different from current flow in a fluid, where current is carried by transport of both positive and negative ions. Seoul's configuration prevents peroxides generated at the anode electrode from reaching the working fluid. However, the electrode/Nafion current is still carried by H+ ions and, thus, the configuration does nothing to inhibit the pH and ionic strength evolution of the fluid in the inner reservoir.
Desiderio discloses a double reservoir bridge design (similar to that used by Wallenborg infra) for use in capillary electrophoresis where the outer reservoir contains a platinum wire electrode. C. Desiderio, S. Fanali and P. Bocek, “A new electrode chamber for stable performance in capillary electrophoresis,” Electrophoresis, 20, 525–528 (1999). The inner and outer reservoirs are separated by a plug of glass wool that serves as the bridge. The object is to minimize the evolution of inner reservoir fluid and thus maintain more constant working fluid properties. The glass wool plug is a porous material having a zeta potential. However, the pore sizes of the conduit and the plug material are sufficiently large that the charge-ratio is negligible. The plug is intended to prevent gross mixing between the outer (electrode) and inner (working fluid) reservoirs.
Ramsey discloses on-chip bridges as a means of making electrical connections in fine microchannels without introducing the gases associated with electrode electrolysis. J. M. Ramsey, S.C. Jacobson, C. T. Culbertson and R. S. Ramsey, “Microfabricated interchannel electrical contacts for material transport control,” Micro Total Analysis Systems 2000, A. van den Berg Ed. (Kluwer Academic, Dordrecht, The Netherlands, 2000) pp. 213–216. Ramsey employs an etched glass chip that is bonded to a glass cover using a sodium silicate interlayer. This interlayer acted as a bridge between two adjacent fluid-filled channels on the chip (channel separation of 3 to 10 microns). This type of bridge falls into the selective ion conducting and flow impermeable class. During the bonding process, the sodium silicate mixture (often called water glass) dries out and forms a very fine pored sodium silicate glass (high positive charge-ratio). When wetted this material acts as a solid ionic conductor that, owing to the negative zeta potential of the glass, preferentially transports positive ions. Thus, the current in this material is primarily carried by positive ions, which is quite different from the bulk fluid where the current is carried by a mix of ions.
The electroosmotic flow channels in Ramsey are sufficiently large that the channel charge-ratio is negligibly small whereas Ramsey's bridges have a high positive value of the charge ratio. This leads to a concentration of positive ions (hence increased ionic strength) on the side of the bridge facing the cathode terminal reservoir and a depletion of negative ions (hence decreased ionic strength) on the side of the bridge facing the anode terminal reservoir.
Paul discloses a bridge to make ionic connections to high pressure junctions in electrokinetically pumped systems. P. H. Paul, D. W. Arnold, D. W. Neyer and K. B. Smith, “Electrokinetic pump applications in micro-total analysis systems, mechanical actuation to HPLC,” Micro Total Analysis Systems 2000, A. van den Berg Ed. (Kluwer Academic, Dordrecht, The Netherlands, 2000) pp. 583–590. The bridge allows the electrode to be removed from the working fluid at a junction in a pressurized microchannel. The bridge is formed from a short section of phase-separated and acid-etch glass (e.g. Vycor or Shirasu porous glass). It has nominal 4 nm pores. For the given conditions (nominally 10 mM or less fluid ionic strength) the bridge has very low permeability to pressure- and electroosmotically-driven flow but is subject to a high degree of charge-layer overlap and, thus, ion-selective current transport. The fine-pored glass bridge is highly charge selective and preferentially transports positive ions, owing to the nanometer-scale pores and the high negative zeta potential of the bridge material. Thus, the current in this material is primarily carried by positive ions whereas the current carried in the pump element, based on a predictive model, is carried near-equally by positive and negative ions (Paul shows a silica pump element supplied with nominal pH 7.5 sodium-phosphate buffered fluid). The imbalance in charge fluxes creates a condition where the fluid flowing out of the pump/bridge junction is at a depleted sodium concentration resulting in a lower degree of phosphate ionization. Thus, the working fluid is at a lower ionic strength and a much lower pH than the source reservoir fluid.
Wallenborg describes various types of bridges for mitigating evolution of reservoir fluid in chip-based empty-channel micellar electro-chromatography (see infra for definition of “empty”). S. R. Wallenborg, C. G. Bailey and P. H. Paul, “On-chip separation of explosive compounds—divided reservoirs to improve reproducibility and minimize buffer depletion,” Micro Total Analysis Systems 2000, A. Van den Berg Ed. (Kluwer Academic, Dordrect, The Netherlands, 2000) pp. 355–358. Wallenborg discloses that in a device comprising a microchannel connected between two terminal reservoirs, oscillations in both current and flowrate are observed. By replacing each terminal reservoir with two reservoirs in series connected with a bridge, the oscillations are significantly reduced with nano-porous bridge materials (specifically: 4 nm pore Shirasu porous glass, 4 nm pore Vycor porous glass, or a nano-porous polymer monolith). However, the use of the bridge introduces a systematic time-variation in ionic strength and hence variations in conductivity and electroosmotic mobility. A larger pore glass material (specifically 70 nm pore Shirasu porous glass) reduces the variation in ionic strength. The small-pored media introduces ion-selective current transport through the bridge and hence the variation in fluid conductivity. This effect is reduced but not eliminated using the larger-pored media that also allows electroosmotic flow. In very fine pored bridge materials the fractional selectivity to current-driven charge transport by ions of one sign is about unity, whereas in the bulk working fluid, this same selectivity is generally about 10% or less owing to minor differences in ion mobilities.
Gan describes the use of a thin cellulose-acetate membrane as a bridge-like structure to isolate fluid in direct contact with the electrodes from fluid flowing in an electroosmotic pump driven by current supplied from the same electrodes. W. Gan, L. Yang, Y. He, R. Zeng, M. L. Cervera and M. de la Guardia, “Mechanism of porous core electroosmotic pump flow injection system and its application to determination of chromium (VI) in waste water,” Talanta 51 pp. 667–675 (2000) which references Y. Z. He and W. E. Gan, Chinese Patent ZL 97212126.9 (1998). A membrane of this type and structure acts to reduce gross mechanical mixing of the fluids. This type of bridge provides the same effect as the glass wool plug used by Desiderio.
Parce describes the use of bridges (termed by Parce a ‘flow restrictor’ or ‘flow restrictive element’) incorporated into microchannel systems applied to placement of electrodes within the fine channels of the system to avoid electrolysis therein. J. W. Parce, Micropump, U.S. Pat. No. 6,012,902 (2000), col. 8, 11. 5–10. See also J. W. Parce, Micropump, WO99/16162 (1999). Parce describes the flow restrictive element as ‘. . . provided to allow passage of current between the electrodes, while substantially preventing flow . . . ’ Id. at col. 8, 11.36–39. Parce further recites that ‘[i]n at least a first aspect, the flow restrictive element includes a fluid barrier that prevents flow of fluid, but permits transmission of electrons or ions, e.g. a salt bridge.’ Id. at col. 8, 11. 44–47. Parce discloses the following types of bridges: agarose or polyacrylamide gel plugs, Id., col. 8, 1. 47; a series of parallel channels each having a much smaller cross sectional area than the remaining channel structure, to reduce the electroosmotic flow through the side channel (bridge) (for example, the much smaller cross sectional area channels have at least one cross sectional dimension in the range from 0.001 to 0.05 microns when the other channels in the system have a size range of about 20 to 100 microns) Id., col. 8, 11. 49–65; and a side channel (bridge) which optionally includes a plurality of side parallel channels, and also substantially lacks surface charge to reduce or eliminate any electroosmotic flow. Id., col. 8, 1. 66 to col. 9, 1. 2.
Parce also describes a configuration that uses two pumping channels (having substantially different charge magnitude and/or sign from each other) that are connected in electrical series. Id., col. 9, 1.3 to col. 10, 1.12. In this configuration, the difference in zeta potential produces a difference in flowrates that results in production of a pressure at the common junction that is used to induce a pressure-driven flow through a third channel connected to this common junction. The phenomena of pressure generation due to variation of zeta potential along a channel is a well-known process [see for example, J. L. Anderson and W. K. Idol, “Electroosmosis through pores with nonuniformly charged walls,” Chem. Eng. Commun., 38 pp. 93–106(1985)].
Because the pumping channels recited by Parce are very large, the channel charge-ratio is negligibly small. Further the finer side channels (bridge) are described as substantially lacking surface charge, this implies a negligibly small value for the charge ratio. The system described by Parce thus operates in the limit of negligible charge-layer effects hence negligible charge-layer-driven net solute transport.
Dasgupta describes the use of a ‘membrane grounding joint’ made of Nafion ion exchange tubing at the end of an empty silica capillary. P. K. Dasgupta and S. Liu, Apparatus and method for flow injection analysis, U.S. Pat. No. 5,573,651 (1996). The grounding joint acts as a bridge to make an electrical connection to the empty capillary that in turn serves as an electroosmotic-flow-pump (EOF pump). Such a bridge is highly selective to positive ion migration (i.e. substantial positive charge ratio) and therefore not matched with the empty capillary electroosmotic element (i.e. negligibly small charge-ratio). As direct evidence of the effect of this mismatch, Dasgupta observes that if the outlet hydrostatic resistance to an EOF pump is increased, the resulting drop in flow rate is accompanied by a decrease in current at the same applied voltage (in fact, this phenomenon is not observed for any uniform ionic strength fluid). The ionic flux mismatch incurred through the use of a Nafion bridge, produces a decrease in ionic strength at the bridge joint. Under normal operation this fluid is carried downstream. With an increase in outlet hydrostatic resistance and hence an increase in backpressure, some fluid from this joint is backflushed into the pumping element. The introduction of the lower concentration, hence lower conductivity, fluid yields the observed reduced current. Without an ionic mismatch at the bridge, the current would be expected to stay substantially constant as the hydrostatic resistance is varied.
To summarize, prior art bridges in electroosmotic flow systems generally fall into four classes: (1) porous media or a membrane with large pores that allows pressure- and electroosmotically-driven flow but inhibits gross mechanical mixing; (2) non-specific ion conducting and flow impermeable media (e.g. a classic salt bridge); (3) porous media with relatively fine pores that greatly restrict pressure- or electroosmotically-driven flow (e.g. pores of order 5 nm diameter or less); and (4) specific ion conducting and flow impermeable media. In the first and second classes and in the third class for materials without a zeta potential, the charge-ratio is negligibly small and, therefore, the material adds little selective current-driven transport of particular ions. In the third class for media with a zeta potential and in the fourth class, the charge-ratio is substantial and the bridge materials are strongly ion-selective and, therefore, the electrode has been removed from direct contact with the working fluid but the action of the bridge may concentrate select ions, thus evolving the working fluid and possibly creating a condition leading to unsteady state operation.
Most of the prior art in electroosmotically-driven flow systems is in the area of interconnected empty microchannel elements, where the conduit diameter is large enough that flux imbalances are essentially negligible (i.e. negligibly small values of the charge-ratio). Thus, the prior art does not teach stable electroosmotic flow systems or bridges for such systems for the substantial charge ratio regime. In fact, the prior art is predominantly concerned with bridge designs for systems where the charge-ratios are negligible for all elements in the system. Bridge designs for substantial charge-ratio cases (the third and fourth classes supra) actually compound the problem of differential ion flux and hence concentration evolution. Therefore, there is a need for designing stable electroosmotic flow systems that contain elements that are subject to some degree of charge-layer overlap leading to unequal flux ratios.